Sunday, June 04, 2006

Time Travel

Is time travel (logically) possible? Presumably not in the popular sense, whereby one somehow alters the past. That seems blatantly contradictory. I don't even think we can change the future, let alone the past. We can certainly causally influence the future. But the future itself is never altered. It is different from the present, for sure. But it is never different from itself. All this is made clear on the eternalist conception of a static timeline. Granted, it would be logically possible to influence the past in much the same way as we influence the future. But that would mean that the past was "that way" all along. A time traveler may play a role in making the past what it is. But she couldn't make it what it isn't.

An interesting question arises concerning in what sense the time-traveler actually "travels" back into the past. Such descriptions seem to require Multi-Dimensional Time, as the agent progresses (along her personal timeline) into the historical past. First she is now, and then (later) she is earlier. To avoid contradiction, the 'later' and 'earlier' must be indexed to different temporal axes. It is later in her personal time, but earlier in historical time. If that makes any sense.

But I'm not sure that it really does make much sense to speak of a second temporal dimension, or 'personal time'. More plausibly, the static historical dimension is the only temporal dimension that there is. Suppose S travels far back in time, from t2 to t1. A more careful description might be this: at t1, S pops into existence [call this event A] -- fully formed with "memories" and all -- lives for a while, and then dies. At some other point she is born, lives for a while, then at t2 she pops out of existence [call this event B]. Further, event B causes event A. If B hadn't occurred, then neither would have A.

Excepting minor details, that's the full story of S's time travel, explained with only one temporal dimension. There's no real (metaphysical) sense in which S experiences t1 after t2. It merely seems that way to her because at t1 she has pre-existing memories of t2, whereas at t2 she does not have any memories of t1. We may wish to construct a kind of "representational timeline", i.e. the timeline of her life as she represents it. But that doesn't mean that this other temporal dimension really exists, in any mind-independent sense. It's just a mental construction. (We are inclined to read more into it because we fall under the illusion of endurance, thinking that our momentary self "passes through time" yet retains its absolute numerical identity as it exists wholly in each moment.)

For those who aren't fussed with the eternalist picture, here's a more intuitive argument for the impossibility of changing the past from what it was. (But "what it was" when? Again, we seem to require multiple dimensions, but this time in a plainly incoherent sense. There's t1 as it was originally, without any time travellers. Then there's t1 as it was after the time-traveller went back and "changed" it. But t1 is just t1, the moment in itself, which cannot intrinsically change over time. Before t1 is t0, let us say, and after it is t2. There's no sense to be made of a t1 "before you went and changed the past", as compared to the t1 that exists "after" the time-traveller does their work.)

Oops, lengthy digression. The perils of late-night blogging, hmm. Anyway, my intended argument was this: Suppose (for reductio) that you were to go back in time and change the past. Let's say you assassinate Hitler. Who assassinated Hitler, and why? You -- some guy from the future -- did it, because Hitler wasn't assassinated and that made the world turn out badly. But hang on. Now Hitler was assassinated. So when the future comes around, "you" won't exist. At least, not like that. Not with those memories, those motivations. So the original time-traveller no longer exists, and so cannot do anything at all, let alone assassinate Hitler. So who did assassinate Hitler? Your ghost?

By changing the past, you change the future, and thereby prevent your original time-travelling self (the one with the original past) from ever having existed quite as you had. It's not as obvious as killing your infant parents. But the essential logic is the same. You (with properties P) cause a change, which causes you to not have existed with properties P, and so you-with-properties-P cannot have caused the change after all. Paradox.

(Though note that one way out of this, which I've blogged about before, might be the possibility of branching timelines. On that view, you don't really change the past. You simply change location, i.e. which of the multiple alternative histories you are in. But that would seem to require cross-history causation, and it isn't clear whether that's possible. But maybe it could be made to work. Logically, I mean. We'd just have to treat the multiple spatiotemporal regions as tenuously-connected parts of one big "universe". But then it's more like Sliders than genuine time-travel.)

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16 comments:

> A time traveler may play a role in making the past what it is. But she couldn't make it what it isn't.

looking at it ths way, can you even make the present what it isn't?

> But I'm not sure that it really does make much sense to speak of a second temporal dimension, or 'personal time'. More plausibly, the static historical dimension is the only temporal dimension that there is.

you do experience time differently depending on various things. AND some might say time is an illusion anyway, rather like the illusion you point out here.

One might say A caused B? I mean if they always occur in Pairs this would be the normal assumption we would make. For example I could say my typing made the post exist, or the post made my typing exist.

> Though note that one way out of this...

I think your argument here tends towards a different solution – this is the "time traveler is part of the system" way of looking at time travel.

In this way of looking at it you would go back in time do everything you could to kill Hitler but when you returned find absolutely nothing changed. You would fail or maybe discover Hitler was killed but it was kept secret or something along those lines because at t2 your actions had already influenced t2.

You can say “that doesn’t make sense! I could just decide to change the plan” but you have been arguing there is no external timeline and the t1 is already factored in at your t2.As a result in this model the (theoretically potentially) infinite loop feeding in at t1 through to t2 and back to t1 are all part of the history at t2 - so it would turn out as above.

You could build this out of a paradox by either saying

1) you create an initial unstable state (lets say you go back in time and kill yourself) but in the next “oscillation” that would happen a bit differently because you are not there to kill yourself. It bounces back and forth until a solution arises that doesn’t create a change and then in essence that example is reinforced an “infinite” number of times and the odds of you experiencing a paradox state tend towards zero.

2) Nature automatically resolves the paradox even if it requires twisting of other laws of physics (which could well be defined largely the same paradox avoiding principle) because the paradox creates a sort of supreme low energy state - rather like how exposing a vacuum results in air rushing in. I see quantum mechanics as a useful tool for solving that.

Nice example. Certaintly, there are constraints to what time travellers can non-paradoxically change about the past. I'm not sure, though, that you've shown that changing the past is logically impossible. Here are two examples which, I think, avoid paradox.

[A] Presumably, a normal time traveller would have no beliefs about the number of dust particles contained within Saturn's innermost ring. But a time traveller could form the following intention in 2006:

(1) Travel back to 1912 in order to increase the number of dust particles contained within Saturn's innermost ring, at that time, by some fixed amount (say, 10).

I see no paradox in successfully fulfilling (1). To fulfill (1) would bring about a change in in the past relative to when, in 2006, the time traveller formed the intention. And as long as she does not "remember" fulfilling (1) in 1912, there is no reason to think that she could not form the intention to do (1) in 2006. This case is non-paradoxical.

[B] Apparently, Hitler barely survived an assassination attempt in 1944. If he has been a couple of feet closer to the explosive, it would have taken is life, or alternatively if the blast radius had been a couple of feet wider. Let's assume that the physical processes leading up to this explosion were indeterministic. Then, perhaps, a time traveller could change the *propensity* of Hitler's death without changing which events in fact occurred. But to change the propensity of an event in one's past is to change the past. So, it is logically possible to change the past.

Let's get more precise. It is true (in 2006) that

C: A bomb exploded near Hitler in 1944,

but false that

E: Hitler was killed by a bomb in 1944.

Now suppose that Prob(E)=65%. Once again, our time traveller forms the following intention in ignorance:

(2) Travel back to 1944 in order to increase Prob(E) by 30%.

She successfully performs (2) by, say, closing the doors and windows to the room and thus concentrating the bomb blast. (Add whatever science fiction is required to keep the rest of the facts fixed.) Again, I don't see why a paradox would arise in such a case.

I also think there ae a number of theories about how the universe worls that effect it - for example we are assuming a clear linier history where one could instead imagine a composite history (reminisient of quantum mechanics)

Hi Alex - my concern is this. Let Saturn originally have x dust particles. Then the time traveller has this relational property: of coming from a world where Saturn has x dust particles. But then she (with this property) changes the past so that Saturn instead has x+10 dust particles. So then she is born into a world where Saturn has x+10 dust particles. So the change cannot have been brought about by someone from a world where Saturn has x particles. There is no such world any more.

(There remain the dimensional concerns about "when" the past used to be different. Really any kind of "change" at all (internal to a moment) seems incoherent to me. Changes must occur over time. See my old post on changeless time.)

G. - "the "time traveler is part of the system" way of looking at time travel"

Yes, that was the main thrust of my post. I merely presented the "branching timelines" as an alternative view. It shouldn't be treated as the "conclusion" of my earlier (quite different) arguments!

The question here is of causility, and how it propagates.Why do u think causuality allways has to hold??

In the quantum world causuality readily breaks down. Indeed you hear headlines such as 'quantum computer returns answer before being turned on!'.Also need to look at how the time affects the space dimensions. Without thinking to hard, i would treat this befroe u were born problem as simply a local violation of the rules of casuality, much like we can break arrows of time such as entropy on local 'isolated' scales.

Yeh but tough question. But i would try to keep an eye on quantum metaphysics (like Bohm), to see any answers there, and in other issues of casuality such as the measurement problem. Because current ideas about 'time-travel' all rely on quantum violations.

I want to suggest the possibility that the model/metaphor of travel is throwing us off.

I immediately started thinking in regard to the first question that we travel through time "all the time", ending up in t2 after just being in t1.

But then, why travel? It reminds of a Sheryl Crowe song, "Every day is a winding road..." Might be a nice model, but does it actually help us determine what we really want to know about Time?

Another model that we might consider to avoid the "road" is, I don't know... For a joke, how about "eating"? We consume time. Can we consume a moment from the past? Every time we read about it. Sorry, for that, but I thought some lateral thinking might be helpful rather than beating our heads against the wall of "travel."

You may already know about it, but David Lewis has a paper entitled "The Paradoxes of Time Travel" (or something like that) in which he defends the claim that time travel is possible. It's available in one of the more recent collections of his papers, if I recall correctly.

First of all, time travel of some sort is possible in Einstein's general theory of relativity. The distribution of matter and energy creates gravitational effects by curving spacetime. Kurt Goedel figured out that if you had a quite peculiar arrangement of stuff in the universe and a world line of a particle moving long enough, fast enough, and in just the right way, it would intersect itself. that is to say it would bounce off of itself. Hardly, the time machine of science fiction, but time travel of a sort supported by one of our current best theories.

And this doesn't require taking time beyond a single temporal dimension. The question of multiple temporal dimensions has been one that fascinated me. What would it be like to be early in one direction and late in another? The only difference in the physical theories between spatial and temporal dimensions is a negative sign. One could easily flip a sign in, say, Einstein's general theory and see what the resulting theory would look like for a space with two spatial and two temporal dimensions instead of three spatial and one temporal. But there are arguments by some very smart folks that a universe like this would be unstable -- energy, for example, becomes a vector (or directional) quantity and not a scalar or just an amount and this has strange physical ramifications. Max Tegmark of MIT, for example, wrote a piece in the journal "Classical and Quantum Gravity" --http://space.mit.edu/home/tegmark/dimensions.pdf -- where he argues this (a bit technical, but readable for someone without a deep physics background). Of course, for Tegmark, there are anthropic principle ramifications to all of this, so I'm not exactly sure what to make of it without a lot more thought.

I think you have misunderstood my scenario. So, let me lay it out for you one more time.

The 2006 temporal part of the time traveller has two relational properties: [being in a world where every temporal part of Saturn's rings before 1912 has x dust particles], and [being in a world were every temporal part of Saturn's rings after 1912 has x+10 dust particles]. Note, first, that these relational properties are entirely consistent. Note, second, that for the 2006 temporal part of the time traveller were to have these two relational properties, then Saturn's rings where less dense before 1912 than afterwards. If this doesn't constitute a change to Saturn's rings, I'm not sure what would.

I'm not sure I see what your worry is. If changes are just Russellian changes (i.e. things with different intrinsic properties at different times), then you paradox is no problem at all. Maybe your worry is about how a time traveller could bring about genuine change. But why should we think that this should be any different from how we normally bring about changes? My suspicion is that you are assuming implicit assumptions about what is required in order for a genuine change to occur. It would be helpfully if you could let us know what these assumptions are.

Alex, thanks for the clarification. I indeed have no objection to that sort of "change". I take that to be a case whereby one merely has a causal influence on the past. One does not alter the past to be different from itself. (As I wrote in my introduction: "Granted, it would be logically possible to influence the past in much the same way as we influence the future. But that would mean that the past was "that way" all along. A time traveler may play a role in making the past what it is. But she couldn't make it what it isn't.")

So I should clarify that I do indeed allow that that sort of time travel could be possible. What I meant to argue against was the "popular" sense, seen in Hollywood movies ("back to the future", etc.), wherein the time-traveller changes the past so that particular moments are not what they (those very moments) used to be.

Waves that travel back in time interfere destructively, thus preventing anything from happening differently from that which has already taken place (www.arxiv.org/quant-ph/0506027). "If you travel into the past quantum mechanically, you would only see those alternatives consistent with the world you left behind you," says Greenberger.

Richard, if there isn't libertarian free will then one can be free in either a deterministic or 4D world. That is the future can be fixed but I'm still free. Thus backwards causality isn't a problem. I can change the future in the sense that my particular actions help make the future what it is. But the future won't be other than what it is. So there's no trouble with patterns (causality) moving in both directions, much like patterns in a rug.

Most attacks on time travel really just raise paradoxes. But all that entails is that in a coherent universe such choices are impossible.

The problem is that at least some (although I don't think all) of our intuitions suggest that the future is made by our choices and the choices are purely originary with us. Thus the future isn't fixed and we can change it. But this notion of *change* runs into problems if the future is fixed. But all it really is is a bias from our intuitions about free will and what it means to chose.

Since I tend to be very skeptical that appeals to intuition tell us much except about our language and culture I don't think they have much philosophical bearing. So I don't have any real problem with backwards causality. Although I can understand why some might say that this is incompatible with free will. The question, to me, then becomes whether our notion of free will is the right one. So my position is generally called the revisionist approach to free will or responsibility. But I'd be the first to admit it isn't a popular one.

Just a note. I think that it is very useful philosophically to distinguish between psychological time (discussed probably best as a historical start by Augustine) from scientific time (roughly considering time as an object rather than a phenomena). When we conflate the two I think we end up with lots of errors.

Certainly it is the case that our perception of time often doesn't line up with the scientific notion of time.

YEs i agree. these paradoxes all arise just from our psycholigical concepts, and other percieved notions such as our logic of causility. (Which BTW i think is a facisnating area of theology if God is outside time.. why everyone wants to apply causality to him..).

For example, its a paradox that a particle can be at both (X AND Y) AND (X OR Y) - i think i explained it corerectly? Duality of wave and particle and the quantum logic that arises.

These are paradoxes that we have struggle to explain for 50 years, but taken covential epistemology, it is how the universe works. We just have to accept the possiblity of the unlogical.

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